[74X] Fix crash in pat::MET when no uncertainties are added

[blinkseb]

Backport of #11394 for 7.4.x

[cmsbuild]

A new Pull Request was created by @blinkseb (Sébastien Brochet) for CMSSW_7_4_X.

[74X] Fix crash in pat::MET when no uncertainties are added

It involves the following packages:

DataFormats/PatCandidates

@cmsbuild, @vadler, @monttj can you please review it and eventually sign? Thanks.
@gpetruc this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[blinkseb]

Now that #11394 is merged, can this be merged too before the new 7.4.X release? Thanks

[blinkseb]

Any update on this. 7.4.15 was released, and it's still crashing... Thanks

[slava77]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9016/console

[cmsbuild]

+1
Tested at: 794364a
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11413/9016/summary.html